In various technical fields, use has been made of capaciatance detectors which convert the capacitance of a capacitor component attendant on an object to be measured into an electric signal for output, where the capacitance varies according to the physical quantity and physical properties of the measured object. Based on the detected signal delivered from such a capaciatance detector, the physical quantity and physical properties of the measured object are determined.
For example, in Patent Document 1, there is disclosed a fuel mixing ratio detector for detecting the mixing ratio of fuel. The detector includes a resonant circuit which is made up of a winding coil L and a capacitor Cf with its capacitance varying depending on the quantity of fuel. The resonant circuit is supplied with a high-frequency signal from a voltage controlled oscillator via an amplifier and a resistor R. Then, the phase difference between signals appearing at both ends P1 and P2 across the resistor R is detected at a phase comparator for output as a phase difference voltage, by which the fuel mixing ratio is determined.
In Patent Document 2 (FIG. 12), there is disclosed an acceleration sensor for detecting the acceleration of a plumb weight. The acceleration is determined by applying square wave signals Φ1 and Φ2, having phases different from each other, to variable capacitors C1 and C2 via resistors R1 and R2, where the capacitors C1 and C2 vary in capacitance as the plumb weight is displaced. Resulting square wave signals appearing on the upper electrode of auxiliary capacitors CC1 and CC2 are supplied to an EX-OR circuit to detect the difference between the capacitance of the variable capacitor C1 and the capacitance of the variable capacitor C2. The acceleration of the plumb weight is determined in this manner.
In Patent Document 3, there is disclosed a polarization orientation detector for ferroelectric materials which makes use of SNDM (Scanning Nonlinear Dielectric Microscope). That is, this device is designed to locate the probe on the surface of a target ferroelectric material to measure its capacitance Cp immediately underneath the probe, thereby determining the polarization orientation of the ferroelectric material. Such a device allows the polarization orientation of the ferroelectric material to be detected by applying alternating electric field Ep from outside between the electrode, and the ring probe and the probe. The alternating electric field causes the oscillation frequency of the oscillator to vary. The rate of change in oscillation frequency including its sign at this time is determined by the capacitance Cp of the ferroelectric material immediately underneath the probe. Thus, the rate of change in frequency can be two-dimensionally scanned with the probe, thereby detecting the polarization distribution of the ferroelectric material. The change in frequency of the oscillator is determined by being synchronously detected at the frequency of the applied electric field by the PSK demodulator after having been demodulated at an FM demodulator.    Patent Document 1: Japanese Patent No. 3126872    Patent Document 2: Japanese Patent Kokai No. 2007-46927    Patent Document 3: Japanese Patent Kokai No. 2004-127489